chore(release): prepare v1.6.0

[Valtora]

Description

Release preparation for v1.6.0. Bumps docs/VERSION from 1.5.0 to 1.6.0 so the v1.6.0 tag matches the version file validated by the release pipeline.

Minor bump: since v1.5.0 the release includes two new features (note-generation overhaul #114, CLI OAuth server-default degrade chain #99), three path-injection security fixes, and several bug fixes.

The Trivy image gate for this release runs in the release pipeline (CI). No new .trivyignore entries are added here; existing accepted kernel-header CVE entries remain within their review-by dates.

No new dependencies.

Type of change

• Documentation update (version file bump; no code or behaviour change)

Checks run

• Docs validation: python3 scripts/validate_docs.py
• Backend tests / frontend suites: not applicable (version-string-only change; CI runs the full gate)

Migration impact

• No database migration in this PR.

Documentation impact

• Updated docs/VERSION as part of the release. No other guide affected.

Security impact

• No security-sensitive change. The release Trivy gate runs in CI at publish time.

Manual verification

Version bump only. Confirmed docs/VERSION reads 1.6.0 and docs validation passes locally.